Embodiments of a bipolar transistor are disclosed, along with methods for making the transistor. An exemplary transistor includes a collector region in a semiconductor substrate, a base layer overlying the collector region and bound by a field oxide layer, a dielectric isolation layer overlying the base layer, and an emitter structure overlying the dielectric isolation layer and contacting the base layer through a central aperture in the dielectric layer. The transistor may be a heterojunction bipolar transistor with the base layer formed of a selectively grown silicon germanium alloy. A dielectric spacer may be formed adjacent the emitter structure and over a portion of the base layer.